Java EE

In a JPA based DAO class, we usually declare an EntityManager as an instance variable annotated with @PersistenceContext, and use it in the methods of the class. This usage seems to work fine and I just accepted it without further thinking. But recently it turned into a question when I was working on some JPA code that, how could an entity manager be thread safe as an instance variable, while an entity manager does not seem to be meant for sharing among multiple threads?